The present invention relates to a magnetic resonance imaging (MRI) apparatus, and more particularly to a RF (radio-frequency) coil for detecting nuclear magnetic resonance images of two or more kinds of nuclides.
MRI apparatuses are diagnostic imaging apparatuses for medical use, which induce magnetic resonance of atomic nuclei in arbitrary sections crossing a subject and obtain tomograms of the sections from generated magnetic resonance signals. As the magnetic resonance signals, those of hydrogen nuclei (1H) are generally used.
MRS (magnetic resonance spectroscopy) and MRSI (magnetic resonance spectroscopic imaging), which are types of magnetic resonance imaging methods, are used as methods for measuring metabolic conditions in the living bodies. Here, MRS is a method of measuring frequency distribution of magnetic resonance signals emitted by a substance, and MRSI is a method of imaging on the basis of certain specific frequency components of magnetic resonance signals having frequency distribution. In these imaging methods, in addition to the imaging based on magnetic resonance signals of hydrogen nuclei (1H), magnetic resonance signals of atomic nuclei other than those of hydrogen such as those of fluorine (19F), phosphorus (31P), sodium (23Na) and carbon (13C) are also imaged. In order to simultaneously obtain images from magnetic resonance signals of two kinds of different atomic nuclei, the RF coil for irradiating a RF magnetic field for exciting the atomic nuclei and detecting the magnetic resonance signals is required to be tuned to the frequencies of the magnetic resonance signals of the two kinds of atomic nuclei (magnetic resonance frequencies). Such a coil is called double-tuned RF coil.
As conventional double-tuned RF coils, there are those comprising a trap circuit having a capacitor and an inductor connected in parallel and inserted into a loop of a coil, as shown in FIG. 20 (see, for example, Japanese Patent Unexamined Publication (Kokai) No. 6-242202 and M. D. Schnall et al., “A New Double-Tuned Probe for Concurrent 1H and 31P NMR”, Journal of Magnetic Resonance, USA, 1985, 65, pp. 122-129). These double-tuned RF coils are intended to be tuned to two kinds of frequencies of magnetic resonance signals which are significantly differ from each other, for example, those of 1H and 31P, and they are not intended to be used for a case where frequencies to which they are tuned are close to each other. In order to realize double-tuning in these double-tuned RF coils to two kinds of frequencies close to each other, the inductor and capacitor used for the trap circuit must have an inductance of 10 nH or lower and a capacitance of several hundreds pF or higher, respectively. Since it is difficult to manufacture inductors having a small inductance, and such inductors also scarcely allow adjustable range, they are impractical. Furthermore, in capacitors having a large capacitance, it becomes impossible to ignore RF loss of the devices themselves at a frequency of 1 MHz or higher, and there is caused degradation of the receiving sensitivity and transmission efficiency of the RF coils.
As double-tuned RF coils used for a case where two kinds of magnetic resonance frequencies are close to each other, there are double-tuned saddle-type RF coils in which two saddle-type RF coils that resonate at each frequency are perpendicularly disposed (refer to FIG. 21.) and double-tuned RF coils in which capacitor of a birdcage RF coil is partially changed so that the coil should resonate at each frequency (see, for example, Peter M. Joseph et al., “A Technique for Double Resonant Operation of Birdcage Imaging Coils”, IEEE Transactions on Medical Imaging, 1989, 8, pp. 286-294).